The present invention relates to magnets for medical magnetic resonance imaging, and more particularly, to such magnets having magnetic zoom capabilities and an open configuration that enables magnetic resonance imaging during surgery.
Magnetic resonance imaging techniques are currently used to obtain images of various portions of an anatomical region of interest. A magnetic resonance imaging magnet assembly generates magnetic field gradients to spatially encode the nuclear magnetic resonance (NMR) signals from an anatomical region which is positioned in the path of the field gradients. The NMR signals are detected and then processed to obtain images that provide an accurate representation of anatomical features and soft tissue contrast of the region of interest.
Early magnet assemblies for performing magnetic resonance imaging on a patient required that the patient be positioned in a narrow, substantially enclosed gap region. These magnet assemblies induced claustrophobic reactions in the patient and also prevented another person, such as a medical attendant or physician, from having easy access to the patient while a region of the patient was scanned to obtain a magnetic resonance image.
Recently, open type magnetic resonance imaging magnet assemblies have been developed. These open assemblies have a large gap region for receiving a patient, are configured to be less confining and also permit greater access to the patient during scanning. For example, magnet assemblies with open areas on four sides of the patient, such as those described in U.S. patent application Ser. No. 07/993,072, filed Dec. 18, 1992, and U.S. patent application, MRI APPARATUS, Gordon Danby, John Linardos, Jevan Damadian and Raymond V. Damadian, filed Nov. 21, 1997, both assigned to the assignee of the present invention and incorporated by reference herein, have been proposed which provide for imaging volumes large enough to conduct surgery therein.
U.S. Ser. No. 07/993,072 also discloses, magnet assemblies have been configured in the form of a room with only the polar regions of the magnet visible in the room, such as projecting from either the horizontal or vertical walls of the room. These magnet assemblies further reduce claustrophobic stress for the patient and allow others even greater access to the patient during scanning. In particular, these magnet assemblies provide that one or more persons can have access to the patient while the patient is positioned between the poles of the magnet assembly during scanning. This accessibility enables a physician to perform surgical procedures on the patient that are guided by the images obtained from scanning a desired anatomical region of the patient. The images obtained using open magnet assemblies, however, may not necessarily have sufficient resolution to be useful for guiding surgery in an anatomical region, which generally is smaller than the anatomical region that the magnet assembly is scanning.
Therefore, there exists a need for an open magnet assembly for magnetic resonance imaging which allows several persons to have access to a patient while the patient is undergoing scanning and furthermore provides a capability of increasing the resolution of scanning over a more limited region of interest of the patient, as desired, simply and conveniently while maintaining access to the patient substantially unimpeded and without requiring that the patient be moved.
In accordance with the present invention, a magnet assembly for use in medical magnetic resonance imaging provides a sizable gap region in which a patient can be received and allows for substantially unimpeded access to the patient while the patient is undergoing scanning of any region of interest. The magnet assembly has a capability to scan a first relatively large volume region of the patient at a first scanning resolution and the capability to scan a second, smaller volume region of the patient at higher scanning resolutions than the first scanning resolution.
In a preferred embodiment, the magnet assembly comprises a ferromagnetic yoke configured as a frame and conformed to the structure of an ordinary room. The frame includes a pair of opposing vertical ferromagnetic elements and a pair of opposing pole supports, each of which forms one side of the frame, which is the flux return path. The pole supports support respective ferromagnetic poles which face each other and are axially aligned with each other. Each of the poles includes a first body portion which is adjacent to the pole support and has a rectangular box structure. Each of the poles further includes a second body portion which extends away from the first body portion and terminates at a gap facing surface. The second body portion is a trapezoidal box structure which includes opposing walls which extend from and are in the same plane as the longer sides of the rectangular first body portion and tapered walls which extend towards the center of the pole at the same angle with respect to the shorter walls of the first body portion. The facing surfaces of the respective poles are spaced apart to define a gap region therebetween for receiving a portion of a patient and each have a magnet field gradient coil support mounted thereto. The gap region and the tapered walls of the poles which are in proximity to the gap region provide for open access to the patient during scanning.
In one aspect of the invention, means for increasing magnetic flux generation in the gap region is coupled to each of the poles. Such increasing magnetic flux generation means, or magnetic zoom means, allows for higher resolution scanning of a smaller volume region of a patient in comparison to the scanning resolution and the volume region of the patient which would be scanned, respectively, when the magnetic zoom means is not utilized. The magnetic zoom means in the magnet assembly decreases the distance between the facing surfaces of the structures of the magnet assembly which extend furthest from the respective poles into the gap region, or the gap distance of the magnet assembly, during higher resolution scanning and, alternatively, also during scanning without magnetic zoom, without substantially impeding access to the patient.
The magnetic zoom means comprises a mechanical magnetic zoom means or an electromagnetic magnetic zoom means, or both, and either of these magnetic zoom magnetic zoom means can be provided in the magnet assembly axially or non-axially axially symmetrical about the center of the poles. The mechanical magnetic zoom means is a ferromagnetic structure which extends or is extendible from the facing surface of each pole into the gap region. The electromagnetic magnetic zoom means comprises a support containing a distribution of conducting coils which is coupled to the facing surface of each pole and extends or is extendible into the gap region.
In a preferred embodiment of either magnet assembly, each pole includes a hollowed cylindrical region in which a piston formed from ferromagnetic material is received in tight fitting relation to the surface of the pole which defines the hollowed region. The piston is coupled to a magnetic zoom operating assembly which is coupled to the adjoining pole support. The operating assembly can position each of the pistons simultaneously and identically at a plurality of positions extending into the gap region to provide for higher resolution scanning of a more limited volume region of the patient in comparison to the region defined by the facing surfaces of the poles. The facing end surfaces of the pistons define the more limited volume region. The surfaces of the pole and the piston which face each other remain in substantial contact with each other at all times to provide a sufficiently large flux contact area.
In a further preferred embodiment, the hollowed cylindrical region of each of the poles receives a first ferromagnetic piston having a hollowed cylindrical region and a second ferromagnetic piston which is disposed in the hollowed region of the first piston. The first piston is in tight fitting relation to the surface of the pole defining the hollowed region and to the outer surface of the second piston facing the first piston. The first and second pistons are each coupled to the magnetic zoom operating assembly. The operating assembly can independently position each of the first and second pistons simultaneously and identically, at various distances extending into the gap region to provide for higher resolution scanning of a more limited region of a patient and adjustability of the magnet fields within the gap region when the higher resolution scanning is performed. The surfaces of the pole and the first piston which face each other, and the surface of the first piston and the second piston which face each other, remain in substantial contact with each other at all times to provide a sufficiently large flux contact area.
In a further embodiment, a multiple axis patient bed is located in the gap region so that the patient can be positioned at almost any desired angle in relation to the facing surfaces of the poles.
In another aspect of the invention, independent electromagnetic zoom means are positioned within the gap region by a mechanical support means and are separate and independent from the poles of a magnet assembly. The independent electromagnetic zoom means are arranged in the gap region to define a volume region of the patient through which an increased magnetic flux density is directed to provide for higher resolution scanning in that region.